In addition to the signs for a kosher fish mentioned in the Torah, The Tur (Y.D. 83) brings alternate kosher signs for fish that are not accepted by all Poskim. Namely, the head of a kosher fish is not sharp and the fish has a spine.
Can someone explain what these signs are? What is considered a sharp head? What is meant by a spine (I'm assuming it doesn't mean the skeleton)? Pictures would be awesome as well.
If it helps, here's the wikipedia article on Fish Anatomy
I didn't find pictures unfortunately, but sharp heads are just that - like in sharks, while kosher fish have a more rounded head, as their mouths are in the front.
The spine means actual bones, as opposed to no bones, or cartilage. Kosher fish will always be from the bony fish family.
Some more here and here. But the main point I think, as mentioned in the second article, is that these secondary signs were used only when they knew which type of fish were around, but not as general primary signs like the ones stated explicitly in the Torah.
